Star just 16 km across unleashes explosion brighter than Milky Way

This is an artist's rendering of the record-breakingly powerful, superluminous supernova ASASSN-15lh as it would appear from an exoplanet located about 10,000 light years away in the host galaxy of the supernova.

Astronomers believe the supernova, first spotted in June 2015, to be a rare star called a magnetar.

A bit of background: When massive stars collapse, they form either a black hole or a neutron star, the most dense star known (one teaspoon of the stuff would weigh about a billion tonnes). A magnetar is an unusual type of neutron star that has an extremely powerful magnetic field. We’re talking in the realm of a quadrillion times more powerful than Earth’s magnetic field. (Magnetars are kind of scary.)

But this unusual star is more powerful than anything that has ever been considered.

A team of astronomers has discovered the most-luminous supernova ever observed, called ASAS-SN-15lh. At its peak it was almost 50 times more luminous than the entire Milky Way galaxy.

The star is about 200 times more powerful than the average supernova and 570 billion times brighter than our own sun. In fact, it’s so bright that it’s 20 times brighter than all the stars in our Milky Way galaxy put together.

“If it really is a magnetar, it’s as if nature took everything we know about magnetars and turned it up to 11,” said the team’s co-principal investigator, Krzysztof Stanek of Ohio State University.

So how can something that pushes the laws of physics come to exist?

One astronomer, Todd Thompson of Ohio State, believes that it could have come from another rare type of star called a millisecond magnetar. These extremely dense and highly magnetic stars spin at an incredible speed. In this case, the star would have had to spin about 1,000 times a second and turn all that energy into light with an efficiency of 100 per cent.

Because ASASSN-15lh is so distant, it isn’t visible with the naked eye. Astronomers used the All Sky Automated Survey for Supernovae (ASAS-SN, pronounced “assassin”) which uses telescopes from around the world for observations.